Diamond has been widely used as superabrasive grit in sawing, drilling, dressing, and grinding applications because of its hardness. Diamond is also an attractive choice for use in high frequency, high-power, high temperature, or high radiation field applications. This is due to its wide band gap (5.5 eV), unequaled thermal conductivity (20 W/cm.sup.2 -K), high electric field breakdown voltage, high electron saturation velocity, and high electron and hole mobilities.
Further, since synthetic diamond is a lightweight insulator with a thermal conductivity four times that of copper, it represents an ideal high performance multi-chip module substrate. Such use of diamond would not only reduce chip temperatures in conventional 2-D multi-chip modules, but also make it practical to implement ultra high performance, 3-dimensionally interconnected multi-chip modules (see Ser. No. 08/016,367). Thus, there exist many present and potential applications for non-gem and synthetic diamond in the marketplace.
However, in order to use non-gem and synthetic diamonds in industrial applications, a metal coating on the diamond is often required. Depending on the application, the metal coating may need to be several microns thick. This requirement has often been a limitation because of poor adhesion of the metal coating on diamond.
For instance, when diamond is used as superabrasive grit in sawing applications, the grit is typically held in a matrix of nickel, copper, iron, cobalt, or tin, or alloys thereof, by mechanical bonds and the matrix is connected to a tool body. In an attempt to improve grit retention, diamond particles have been coated with carbide forming transition metals, such as titanium or zirconium, by metal vapor deposition. The coating's inner surface forms a carbide with the diamond. A second layer of a less oxidizable metal, such as nickel or copper, can then be applied to protect the inner layer from oxidation.
Tensile testing of double layer coated diamond having an inner layer, such as titanium, and an outer layer, such as nickel, shows that fracturing occurs at the interface between the inner and outer layers. This suggests that the second coating does not bond well with the underlying carbide layer.
In U.S. Pat. No. 3,929,432, Caveney controls the duration of heat treatment in a double coated diamond particle. Using heat treatment after the deposition of the metal coatings, Caveney discloses the formation of a chemical metal-carbide bond between the first metal and diamond, and alloying between the second metal coating and the first metal coating. The patent does not address adhesion between the inner and outer metal coatings when the same metal is used for both coatings.
Chen, in U.S. Pat. No. 5,024,680, further discloses the use of heat treatment in a multiple metal coated diamond grit to improve adhesion between the coatings. A first layer coating, preferably chromium, titanium, or zirconium, forms a chemical metal carbide bond with diamond. A second metal coating of an oxidation resistant carbide former, preferably tungsten or tantalum, is chemically bonded to the first metal layer. A third metal layer coating of an alloying metal, such as nickel, can also be added.
Heat treatment is a preferred step in the process to increase carbidization of the second layer and increase the bond strength between the first and second layers. In Chen's process, the heat treatment is done after the second metal is deposited over the first metal. There is no heat treatment in between the deposition of the initial coating and the second coating. Further, in Chen each coating is a different metal.
Other applications of diamond that require metal coatings are electronic devices. Such devices use a solid-state reaction process for forming adherent ohmic contacts on diamond (see Roser, et al., High Temperature Reliability of Refractory Metal Ohmic Contacts to Diamond, J. Electrochemical Soc.,139(7), 2001-2004 (July 1992)).
The process consists of the deposition of a thin layer of carbide-forming metal on diamond followed by the deposition of a gold cap layer to protect the metal from corrosion. The carbide-forming metals that have been utilized in forming these contacts include molybdenum, titanium, vanadium, and tantalum. After deposition, the contacts are annealed at high temperature in a purified hydrogen environment. Along with the other processes already mentioned, the disadvantage of this process is that it is limited to thin films on the order of a few thousand angstroms.
Recently, diamond has shown promise as a substrate in 3-dimensional multi-chip modules. The ability to produce high density packing of components on diamond substrates is dependent on a technique to pattern and interconnect traces on both sides of the diamond. This can be accomplished by fabricating metallized vias in the diamond (see copending application Ser. No. 08/016,367).
In such an application, a thick coating of a single metal is required because the vias need to be completely filled with a metal which demonstrates good electrical and thermal conductivity, reasonable thermal expansion match to diamond, and strong adhesion to diamond. These requirements generally limit the metallization to refractory metals.
It has been shown that direct deposition of a thick coating of tungsten on diamond by chemical vapor deposition, of about 15-20 microns thick, displays poor adhesion to the diamond. The deposit cracks and peels from the substrate. This occurs even after heat treatment of the substrate and metal deposit.
Presently, the adhesion on diamond of two or more coatings of varying metals can be improved by heat treatment. However, there is still a need to develop a process that provides a tightly adherent thick coat of a single metal on diamond.
Until now, the adhesive strength of tungsten on diamond has generally been limited to a few hundred pounds per square inch. This may be insufficient for some applications. Thus, there is also a need to develop a process that significantly increases the adhesive strength of metal coatings on diamond.